Norisoboldine Alleviates Isoproterenol‐Induced Myocardial Ischemic Injury via the <scp>TLR4</scp>–<scp>MyD88</scp>‐Dependent <scp>NF</scp>‐<scp>κB</scp> Activation Pathway and Modulation of L‐Type Calcium Channels

Norisoboldine (NIB) displays beneficial effects on cardiovascular diseases, although its protective role and underlying mechanisms in myocardial ischemia (MI) injury remain elusive. The aim of this study is to explore the potential cardioprotective mechanism of NIB on MI injury caused by isoproterenol (ISO). We administered NIB to SD rats at 20 and 40 mg/kg daily for 7 days in this study; this was followed by an ISO injection to induce MI injury. Parameters such as electrocardiogram readings, heart rate, serum concentrations of creatine kinase (CK) and creatine kinase-MB (CK-MB), levels of inflammatory markers, some histopathological assessments and oxidative stress markers were evaluated. We conducted Western blot analyses to evaluate protein expression related to apoptosis and the TLR4-MyD88-mediated NF-κB activation pathway. The L-type Ca²⁺ current (ICa-L) and contraction of isolated ventricular cells from rats were identified using patch-clamp methods and the IonOptix detection system. The treatment with NIB resulted in improvements in heart rate and ST-segment changes, a reduction in CK and CK-MB levels, the restoration of superoxide dismutase, catalase and glutathione levels and a decrease in malondialdehyde accumulation. Furthermore, NIB reduced the expression of inflammatory markers, lowered Ca²⁺ levels and reactive oxygen species production and improved myocardial tissue morphology. It also countered ISO-induced alterations in apoptosis and the TLR4-MyD88-dependent NF-κB activation pathway. Additionally, NIB considerably attenuated ICa-L and reduced the contractile function of cardiomyocytes. These results suggest that NIB effectively mitigates ISO-induced MI injury through anti-inflammatory, antioxidative, and anti-apoptotic mechanisms, potentially involving the TLR4-MyD88-dependent NF-κB activation pathway and calcium balance.